1. Field of the Invention
The present invention relates to an apparatus with an optical functional device having a special wiring electrode, such as a surface emitting semiconductor device or a light receiving device, whose fabrication is easy, whose yield is high, and which can be suitably constructed as a two-dimensional array or the like. This invention also relates to a fabrication method of the above apparatus and apparatuses using the above apparatus, such as an optical transceiver apparatus, an optical interconnection apparatus and an optical recording apparatus.
2. Related Background Art
Recently, development of a solid-state light emitting device of a two-dimensional array type has been desired for purposes of its applications to a large-capacity parallel optical information processing, a high-speed optical connection, a high-speed optical recording and a panel-type display apparatus. Low cost, low consumption of an electric power, high productivity, and high reliability are required to achieve those applications. A variety of materials for such a solid-state surface emitting device have been studied and developed. It has been found that single-crystal semiconductors are notably suitable for reliability. In particular, development of a surface emitting device using compound semiconductors has been energetically advanced.
Among light emitting devices, a laser diode (LD) including reflection mirrors at its opposite ends is quite excellent in a light emitting efficiency, compared to devices using spontaneous emission. Therefore, the electric power consumption can be greatly lowered when those LDs are arranged in a two-dimensional form. In light of these facts, development of a VCSEL (vertical cavity surface emitting laser) has been actively advanced in recent years.
With the VCSEL, devices have been developed over a range from a blue color at a wavelength of about 400 nm to a communication wavelength band of 1.55 xcexcm. Studies have been made in material series, such as AlGaN/InGaN series on a sapphire substrate, InGaAlP/InAlP and InGaAs/AlGaAs series on a GaAs substrate, and InGaAs/InGaAsP series on an InP substrate.
A fundamental structure of the two-dimensional arrayed VCSELs is illustrated in FIG. 1. Laser light is emitted perpendicularly to a substrate 1001. Each device is provided with highly-reflective coatings 1002 and 1004 of over 99% reflectivity at opposite ends of epitaxially-grown layers with a thickness of about several microns. A multiplicity of alternately-layered layers with different refractive indices and a common xcex/4 thickness are used as the reflective mirror. The materials are generally dielectric or epitaxially-grown semiconductors (see, for example, an AlAs/GaAs mirror as disclosed in ELECTRONICS LETTERS, 31, p.560 (1995).
Regarding other elements shown in FIG. 1, reference numeral 1003 designates an active layer, reference numeral 1005 designates an insulating layer, reference numeral 1006 designates an electrode on the epitaxial layers, reference numeral 1007 designates a laser functional portion, reference numeral 1008 designates an electrode on the laser substrate side, reference numeral 1009 designates a burying layer, reference numeral 1010 designates a window region formed in the electrode 1006, and reference numeral 1011 designates a laser cavity.
Several mounting or packaging methods of the VCSEL have also been proposed so far. Japanese Patent Laid-Open No. 8-186326 (1996), for example, discloses a mounting method in which the laser is thermally and electrically connected to a package and heat-sink substrate that is transparent to laser light, as illustrated in FIG. 2. In this case, a wiring electrode is formed on a portion of the heat sink corresponding to a portion of the VCSEL, and the electric connection is achieved by a solder heating or an ultrasonic-wave bonding between Au portions at the VCSEL portion.
In FIG. 2, reference numeral 1110 designates a resin mold body, reference numeral 1111 designates a laser electrode, reference numeral 1112 designates a laser substrate, reference numerals 1113 and 1117 designate reflective layers, reference numerals 1114 and 1116 designate cladding layers, reference numeral 1115 designates an active layer, reference numeral 1118 designates a current blocking layer, reference numeral 1119 designates a contact layer, reference numeral 1120 designates a package window (also referred to, herein, as a heat sink window), and reference numeral 1121 designates an electrode on the package window side.
Further, Japanese Patent Laid-Open No. 6-237016 (1994) discloses another mounting method in which a wiring electrode is formed on a substrate 1201 with an electronic circuit, the wiring electrode is electrically connected to a VCSEL 1203 and an optical fiber 1210 is inserted into a hole formed in a VCSEL substrate 1202. Herein, a transistor 1204 is arranged under the VCSEL 1203, and a cathode of the VCSEL 1203 is connected to a collector of the transistor 1204 as illustrated in FIG. 3.
Regarding other elements shown in FIG. 3, reference numeral 1205 designates an emitter electrode, reference numeral 1206 designates a base electrode, reference numeral 1207 designates an anode electrode, reference numeral 1208 designates an insulating layer, reference numeral 1209 designates a guide hole, and reference numeral 1211 designates an adhesive.
Further, Japanese Patent Laid-Open No. 9-15459 (1997) discloses a mounting method of arrayed VCSELs in which a wiring 1304 is formed on a support substrate 1305 and the wiring 1304 is connected to each VCSEL 1302 as illustrated in FIG. 4.
In FIG. 4, reference numeral 1301 designates a semiconductor substrate, reference numeral 1303 designates a guide for an optical fiber, reference numeral 1306 designates an optical fiber tape, reference numeral 1307 designates a core of the fiber, reference numeral 1308 designates a clad of the fiber, reference numeral 1309 designates a core wire of the optical fiber, reference numeral 1310 designates a coating material, reference numeral 1311 designates a light beam, and reference numeral 1312 designates a length of an exposed portion of the core wire 1309 of the optical fiber.
In those prior art mounting methods, VCSELs with an electrode pad formed on an upper surface (i.e., a surface of epitaxial layers) of each VCSEL are bonded to a wiring substrate with an electrode pad corresponding to each VCSEL while electrical connections between pairs of these pads are maintained. In such mounting methods, however, an alignment precision is required when the pads on the VCSEL and the wiring substrate are bonded, especially where the density of a two-dimensional array of VCSELs is great or where the electrode pad is made small to secure a preferable rapid response. The reason therefor is that the pads must be precisely positioned and in addition thereto the alignment and bonding must be performed such that the pad would not be brought into contact with other wirings on the wiring substrate. As the density increases, such undesired contact is more likely to occur since a number of wirings are formed between the pads. Accordingly, in such methods wherein mutually-bonding faces cannot be directly monitored, the alignment is difficult to achieve and yield and productivity decrease.
Further, the uses of solder to perform bonding cause several problems. For example, the melted solder flows into a window through which laser light emerges and blocks the laser light. Also, a resistance of an ohmic contact with the VCSEL rises since the solder melts an Au electrode. Additionally, when the Au electrodes are directly bonded by ultrasonic waves, the VCSEL is likely to be damaged and less effective if the bonding point is close to the VCSEL.
Further, in the prior art structure of FIG. 2, when light emerging from the VCSEL through the heat sink window 1120 is spatially transmitted or coupled to an optical fiber, a lens is needed and the optical system thus becomes complicated, leading to an increase in cost.
In the prior art structure of FIG. 3, since the VCSEL substrate 1202 needs to be transparent to laser light, a usable wavelength band of the laser is limited. In FIG. 3, though the substrate 1202 appears to be completely removed to form the guide hole 1209, it is actually difficult to completely remove a portion of the substrate 1202 between the VCSEL 1203 and the guide hole 1209. Therefore, the VCSEL substrate 1202 must be transparent to the laser light as noted above. Further, FIG. 3 shows a structure of the guide hole 1209 formed in the VCSEL substrate 1202 to couple the laser light to the optical fiber 1210. There are, however, problems in that an alignment of the guide hole 1209 is difficult to perform, coupling loss cannot be reduced since the laser light is coupled to the optical fiber 1210 without any lenses and that the laser is likely to be injured when the optical fiber 1210 is inserted.
Furthermore, when a laser is actually driven by a constant driving current only, it is difficult to maintain an output power of the laser at a constant magnitude due to adverse influences of its ambient temperature and history. Therefore, an automatic power control (APC) is normally employed. FIG. 5 illustrates a structure therefor. In FIG. 5, laser light is generally emitted from opposite sides of two cavity mirrors of a laser diode (LD) 2200. One of the light outputs is monitored by a photodiode 2201, so that the laser output is maintained at a constant magnitude by comparing the monitored value with a reference voltage 2202 by a differential amplifier 2203 and negatively feeding its comparison result back to a driving current of the laser 2200. In FIG. 5, reference numeral 2204 denotes an LD current controlling circuit, reference numerals 2205, 2206 and 2207 denote resistors and reference numeral 2208 denotes an optical fiber.
Herein, in the case of an end-facet emitting laser 2309 as illustrated in FIG. 6, generally the laser 2309 and a photodiode (PD) 2310 are mounted on a common support and the laser output is monitored. Regarding other elements shown in FIG. 6, reference numerals 2311 and 2312 denote lead wires, reference numeral 2313 denotes a cap, reference numeral 2314 denotes a window plate, and reference numeral 2315 denotes a stem. In the case of VCSEL, however, there is a light output from only one of the cavity mirrors since its entire substrate is bonded as illustrated in FIG. 2.
Accordingly, when the APC is carried out in the VCSEL, it is necessary to divide one output into two by a prism and optically create monitoring light. Hence, the optical system becomes complicated and the number of components increases, leading to an increase in cost. Further, an actual output power decreases since a portion of the light output is taken out for monitoring, and hence the amount of a laser driving current needs to be increased in order to obtain a necessary output power. Its power requirements thus rise inevitably.
An object of the present invention is to provide an apparatus with an optical functional device having a special wiring electrode, such as a surface emitting semiconductor device, a light receiving device, whose fabrication is easy, whose yield is high, and whose cost can be relatively low, and a fabrication method of the above apparatus and apparatuses using the above apparatus, such as an optical transceiver apparatus, an optical interconnection apparatus and an optical recording apparatus.
An apparatus with an optical functional device for achieving the object of the present invention includes a first substrate, a first optical functional device with a functional portion provided on the first substrate, a first wiring electrode for injecting a current into or applying a voltage to the functional portion of the first optical functional device, a second substrate, and a second wiring electrode. Herein, the first wiring electrode is formed on the first substrate and includes a first portion electrically connected to the functional portion, a first extension portion extending from the first portion to an outside of the functional portion and a first pad portion connected to the extension portion outside the functional portion, and the second wiring electrode is formed on the second substrate and includes a second pad portion and a second extension portion extending from the second pad portion. The first substrate and the second substrate are bonded to each other with the first pad portion and the second pad portion being electrically connected to each other. Due to such a structure, an alignment precision and damages to the optical functional device at the time of bonding can be reduced or eliminated, irrespective of pitches between the optical functional devices (where plural devices are arranged) and device""s size.
Based on the above fundamental structure, the following specific structures are possible with the following technical advantages.
At least one of input light and output light travels into and from the first optical functional device on a side of the first substrate. In this case, a portion of the first substrate corresponding to the functional portion of the first optical functional device may be removed to form a window region, which achieves the light input and output on the first substrate side independently from a wavelength treated by the first optical functional device. Alternatively, the first substrate may be formed of material transparent to light which is treated by the first optical functional device.
At least one of input light and output light travels into and from the first optical functional device on a side of the second substrate. In this case, the second substrate may be formed of material transparent to light which is treated by the first optical functional device.
Further, a guide unit for guiding the above at least one of input light and output light into and from the first optical functional device can be provided at a portion of the second substrate corresponding to a location of the first optical functional device. An efficiency of optical coupling of the light input or output to the first optical functional device can be increased due to this structure. In this case, the guide unit may be a microlens or a Fresnel lens formed in the second substrate. The efficiency of optical coupling can be increased due to a light collimating and condensing function of such a lens. Further, the guide unit may be an optical fiber fixed to the second substrate. In this case, a hole may be formed in the second substrate, and the optical fiber may be fixed into the hole. Thus, the first optical functional device and the optical fiber can be integrally arranged.
Further, a third substrate with a hole bonded to the second substrate may be provided, and the an optical fiber may be fixed in the hole.
The first optical functional device can be a surface emitting light-radiating device for emitting light perpendicular to the first substrate (typically, a vertical cavity surface emitting laser (VCSEL) with an active layer sandwiched between a pair of reflective mirrors). The first optical functional device can also be a photodetector for converting received light to an electric signal.
Further, a photodetector can be provided on the second substrate when the first optical functional device is a surface emitting light-radiating device for emitting light perpendicular to the first substrate. In this structure, a light output of the light-radiating device can be monitored by the integrally arranged photodetector without attenuating the light output of the light-radiating device and increasing the number of components.
The photodetector may be positioned oppositely to the surface emitting light-radiating device to receive light from the surface emitting light-radiating device. The photodetector may also be positioned oppositely to the surface emitting light-radiating device to receive both external light and light from the surface emitting light-radiating device. In this case, the photodetector may be positioned on a face of the second substrate opposite to a face of the second substrate bonded to the first substrate to receive the external light. The photodetector may also be positioned on a portion of the second substrate, at which material of the second substrate is removed to form a window region, to receive the external light. High performance can be attained when the photodetector can also receive the external light.
The second pad portion of the second wiring electrode is preferably formed outside the photodetector.
Further, a third wiring electrode may be formed on the second substrate for the photodetector. The third wiring electrode includes a third portion electrically connected to the photodetector, a third extension portion extending from the third portion to an outside of the photodetector and a third pad portion connected to the third extension portion outside the photodetector.
The photodetector may be a pin-photodiode, and an electrode of the pin-photodiode is the third wiring electrode while another electrode of the pin-photodiode is formed on an entire face of the second substrate opposite to a face of the second substrate on which the photodetector is provided. Alternatively, the photodetector may be a metal-semiconductor-metal (MSM)-photodiode, and an electrode of the MSM-photodiode is the third wiring electrode while another electrode of the MSM-photodiode is another wiring electrode drawn from the MSM-photodiode to an outside of the MSM-photodiode.
Further, a plurality of arrayed photodetectors may be provided on the second substrate corresponding to a plurality of the first optical functional devices provided on the first substrate.
The photodetector can receive light from the surface emitting light-radiating device and feedback controls the surface emitting light-radiating device based on the received light such that a light output of the surface emitting light-radiating device can be stabilized.
The photodetector can receive both external light and light from the surface emitting light-radiating device and feedback controls the surface emitting light-radiating device based on the received light such that a light output of the surface emitting light-radiating device can be stabilized and that the surface emitting light-radiating device can act as an optical inverter.
Further, the first pad portion and the second pad portion may be bonded with solder, anisotropic electrically-conductive adhesive, or electrically-conductive adhesive. The first pad portion and the second pad portion may also be bonded by pressing the first and and second pad portions against each other.
A metal bump for radiating heat generated by the functional portion of the first optical functional device to the second substrate may be provided. The metal bump is placed between the functional portion and the second substrate, and said metal bump being electrically independent.
Resin may be packed in a space created between the functional portion of the first optical functional device and the second substrate to strengthen a mechanical bonding between the first and second substrates.
Further, the second substrate with the second wiring electrode may be mounted to a package or a print-circuit board directly or through another substrate with a wiring electrode.
The first substrate may be entirely bonded to an inner surface of a package acting as a heat sink.
The second substrate can be a substrate of a semiconductor single crystal, and the second substrate may include an electronic functional device, a driver for the electronic functional device and a controller for the electronic functional device integrated on the second substrate.
The first substrate may include a groove structure for preventing a short circuit through an end facet of the first substrate. The groove structure is formed around the functional portion of the first optical functional device.
Further, a plurality of arrayed first optical functional devices may be provided in an integrated form on the first substrate, and the plurality of first pad portions respectively connected to the plurality of arrayed first optical functional devices may be respectively extended from the plurality of first portions to a region outside of the first optical functional devices. In this case, a groove for preventing optical, electric and thermal interference between the plurality of first optical functional devices may be formed between the plurality of first optical functional devices.
The first wiring electrode may be formed for each first optical functional device such that each first optical functional device can be independently driven. The first wiring electrode may also be formed such that the first optical functional devices can be driven in a matrix manner.
A substrate structure for achieving the object of the present invention includes a substrate, an optical functional device with a functional portion provided on the substrate, and a wiring electrode for injecting a current in or applying a voltage to the functional portion of the optical functional device. Herein, the wiring electrode is formed on the substrate and includes a portion electrically connected to the functional portion, an extension portion extending from the portion to an outside of the functional portion and a pad portion connected to the extension portion outside the functional portion.
A substrate structure for achieving the object of the present invention includes a substrate and a wiring electrode formed on the substrate and including a pad portion and an extension portion extending from the pad portion. The substrate structure also includes an optical functional device and another wiring electrode formed on the substrate for the optical functional device. Herein, the another wiring electrode includes another portion electrically connected to the optical functional device, another extension portion extending from the another portion to an outside of the optical functional device and another pad portion connected to the another extension portion outside the optical functional device, and wherein the pad portion of the wiring electrode is formed outside the optical functional device.
A fabrication method of the above apparatus with an optical functional device and an optical fiber for achieving the object of the present invention includes the steps of:
forming the surface emitting light-radiating device and the first wiring electrode on the first substrate;
forming the second wiring electrode on the second substrate;
bonding the first substrate and the second substrate to each other;
injecting a current into the surface emitting light-radiating device to oscillate the surface emitting light-radiating device;
aligning a photomask with one of the second substrate and the third substrate bonded to the second substrate using the oscillated light as an alignment mark;
performing a patterning on one of the second substrate and the third substrate using the photomask to form a pattern on one of the second substrate and the third substrate; and
etching the hole in one of the second substrate and the third substrate using the pattern.
Further, an optical recording apparatus for achieving the object of the present invention includes the above apparatus with an optical functional device, and a unit for projecting signal-carrying light from the apparatus with an optical functional device on a recording medium.
An optical transceiver apparatus for achieving the object of the present invention includes an optical transmitter of the above apparatus with an optical functional device, and an optical receiver. The optical functional device includes a photodetector provided on the second substrate. In this case, the above apparatus with an optical functional device can act as both the optical transmitter and the optical receiver. The optical transceiver apparatus can perform an inter-board parallel transmission and a signal processing and is constructed as an optical interconnection apparatus.
These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings.